Equatorial Annual Cycle Shang-Ping Xie IPRC/Met, University of Hawaii Ocean University of China PowerPoint file available at References Mitchell, T.P. and J.M. Wallace, 1992: The annual cycle in equatorial convection and sea surface temperature. J. Climate, 5, Xie, S.-P., 1994: On the genesis of the equatorial annual cycle. J. Climate, 7,

SST 165W, 20N 90W, Eq Galapagos SST and Precipitation

Calendar Month Nino3 std dev ENSO’s Seasonal Phase Locking

uvT Equatorial Annual Cycle  Why annual?  Why Strong in the east?  Why propagate westward?

Lukas and Firing (1985, J. Phys. Oceanogr.) xx xx yy yy  = A(x) e i[  t -  (x) ] A(x)  (x) cf. Horel (1982, Mon. Wea. Rev.)

Mar-Apr Aug-Sept SST, Precipitation and Surface Winds

August-May Difference Sea surface height (cm) cf. Mitchell and Wallace (1992) Sept-Mar SST & Wind Diff (COADS)

Buoy Measurements at 110W, Eq. From Xie (1994, JC)  Why is the annual cycle in h small in the Eq Pacific?

1D Ocean (coupling) Simple Theory of Equatorial Annual Cycle Linearization  How to make this coupled equation unstable? Hint: atmospheric model.

 Northward displaced ITCZ ( >0)  Annual frequency (V’);  Tilt of the thermocline H(x)  Stronger annual cycle in the east;  Prevailing easterlies ( <0)  Westward phase propagation. (Xie 1994, J. Climate, p.2008) cf: Liu & Xie (1994, JAS) Evaporation: E= Upwelling: Xie 1998, J. Climate, Eq. (2.5), p < <0 cf: Giese & Carton (1994, JC); Chang (1996, JC)

0 |V|  Annual Annual V’ in both cases

Temperature along equator SST’ & u’ at Eq V eq - +

Xie 1994, J. Climate Model Results

Response to cross-equatorial winds Philander & Pacanowski (1981, Tellus)

SST Wind Cloud SST: Mean & Annual Harmonic

Sensitivity to the length of year SST xx yy 1 yr = 12 mon1 yr = 18 mon Giese and Carton (1994, JC)

Control Flux corrected Li and Hogan (1999, JC)

Control Annual-mean correction Seasonal correction Obs Li and Hogan (1999, JC) Improved the mean state (asymmetrical about the equator)  Annual cycle on the equator

Gordon et al. (2000, JC) Yu and Mechoso (1999, JC) Prescribed observed cloudiness in a CGCM  Improved the mean state (asymmetrical about the equator)  Seasonal forcing by cloud

0 o, Eq 110 o W, Eq Pacific Atlantic

Depth (m) Equatorial Annual Cycle in the Atlantic Ocean dynamics play a more important role Houghton (1983, JPO, p. 2070)

Annual-mean March-April July-August

I year Annual cycle in the equatorial oceans Mitchell and Wallace (1992) Role of Air-sea interaction

Seasonal cycle of equatorial zonal wind: (1) Local air-sea interaction U eq (m/s) April June Longitude CTL run APR run CTL-APR

Surface wind & precip Monsoon Effect June-April diff in APR run with cold tongue removed ITCZ Eq. Equatorward momentum advection Mean

Monsoon  Cold tongue Cold tongue effect CTL-APR anomalies in June Surface wind (m/s) and precipitation (mm/day) Monsoon effect June-April diff in APR run with cold tongue removed Okumura and Xie (2004, J. Climate)

 Northward displaced ITCZ  Annual frequency (V’)  Tilt of the thermocline  Stronger annual cycle in the east  Prevailing easterlies  Westward phase propagation  While secondary in the eastern Pacific, ocean dynamics are important for equatorial annual cycle in the Atlantic.  Atlantic equatorial cycle is strongly influenced by continents and African monsoon in particular. Summary

Eq IO seasonal cycle: uncoupled in the central basin SST xx uouo Wyrtki jets SST  cloud: 1 yr Zonal wind & current: 0.5 yr

Cane and Sarachik (1981, JMR); Cane and Moore (1981, JPO) Cn= 163 cm/s, m = 1, Cn = 82 cm/s, m = 2, Basin-mode resonance at the semi-annual period Jensen (1993, JGR, ); Han et al. (1999, JPO, 2191-) T = 0.5 year ( period) L = 5, 6327 km (basin width) K K R wind

COADS Zonal Wind (m/s) Nov AVHRR SST (C, 5-day, 85-99) Nov easterly acceleration and SST response COADS SST (C) TOPEX/Poseidon SSH (cm) Nov

Thermocline depth control of SST variability Rms SST ( ) Nov 0 Jun T/P SSH (cm) 20W 40W Yuko Okumura, U of Hawaii